1. Field of the Invention
The present invention relates to 180 degree hybrid couplers.
2. Description of the Related Art
A Hybrid coupler is a passive device that has a wide range of applications in microwave circuits. A Hybrid coupler comprises four RF ports, wherein two of the four RF ports are input ports, and two of the four RF ports are output ports. Ideal hybrid couplers are perfectly matched on all four ports; furthermore, the two input ports of an ideal hybrid coupler are mutually isolated and the two output ports are mutually isolated.
Hybrid couplers are often employed in microwave circuits for splitting a pair of input signals into two output signals; hybrid couplers can also be used for combining a pair of RF signals.
Broadly speaking, there are two types of hybrid coupler, a 90 degree hybrid coupler and a 180 degree hybrid coupler. When an RF signal is fed to either of the two input ports of a 90 degree hybrid coupler, there is a phase difference of 90 degrees between the signals at the two output ports of the coupler. For a 180 degree hybrid coupler, when an RF signal is fed to one of the two input ports, the signals at the two output ports have the same phase; on the other hand, when an RF signal is fed to the other of the two input ports, the signals at the two output ports have a phase difference of 180 degrees. The outputs and inputs of a hybrid coupler can be interchanged, and the phase relations described above still apply.
In addition to the phase relationship between the signals at the four ports of a hybrid coupler as described above, there is a relationship for the power of the signals at the output ports. For example, a −3 dB hybrid coupler divides the power of a signal at either input equally between the two output ports.
Signal division between output ports can be intentionally made unequal for some applications; however the most common applications of 180 degree hybrid couplers is feeding signals to two identical circuits, or combining the signals from two identical circuits. For these applications in particular, the equal division or combining of signals is normally required.
A number of different technologies can be employed for the fabrication of hybrid couplers. For example, microstrip technology, where metal tracks forming transmission lines are fabricated on the top side of a dielectric layer and where the bottom side of the dielectric layer is substantially covered with a metal ground plane (terms of orientation are used for convenience and refer to the orientation of the devices as seen in the drawings, and do not imply any particular orientation in use).
A conventional microstrip 180 degree hybrid coupler is illustrated in FIGS. 1 and 2, FIG. 1 being a plan view of the coupler geometry and FIG. 2 being a cross-section taken on line II-II of FIG. 1. The coupler comprises a microstrip metal ring 10 on the top side of a dielectric layer 12 whose bottom side is covered with a metal ground plane 14 (it will be appreciated that only the top metal ring 10 is shown in FIG. 1). The ring 10 has perimeter of 3λ/2 with four ports connected around the ring, each port 1 to 4 being separated by λ/4, λ/4, λ/4 and 3λ/4 respectively from its immediately preceding neighbour (λ is the wavelength of the operating frequency of the coupler). When operated as a combiner with input signals applied at ports 1 and 3, the sum of the inputs will be formed at port 2, while the difference of the inputs will be formed at port 4. Hence, ports 2 and 4 are referred to as the sum (Σ) and difference ports (Δ), respectively. A more detailed description of conventional hybrid couplers can be found in Pozar D: “Microwave Engineering”, Second Edition, John Wiley & Sons, New York, 1998.
One of the applications of a 180 degree hybrid coupler as described above could be, for example, in monopulse radar systems where signals from two identical antennas are connected to the hybrid coupler input ports and where sum (Σ) and difference (Δ) signals from the output ports of the hybrid coupler are amplified, demodulated and processed to obtain the information about target azimuth.
A recently introduced implementation of a microstrip hybrid coupler is described in Myun-Joo Park and Byungje Lee: “Coupled Line 180 Deg Hybrid Coupler”, Microwave and Optical Technology Letters, Vol. 45, No. 2, Apr. 20, 2005. FIG. 3 is a plan view of the coupler and FIG. 4 is cross-section taken on line IV-IV of FIG. 3. This implementation comprises a cascaded pair of quarter wavelength edge-coupled directional couplers 16 and 18 respectively, where one of the connections between the pair of directional couplers is made by direct connection 20 and the other connection between the pair of directional couplers is made using a loop of microstrip line 22 that introduces a phase shift of 180 degrees at the operating frequency of the coupler. It can be shown that this topology has the same electrical properties as the 180 degree hybrid coupler of FIGS. 1 and 2. It can also be shown that for −3 dB coupling between either input and either output of the hybrid coupler (equal power splitting between the output ports) the coupling ratios of each of the individual directional couplers should be −7.7 dB.